How US can force this punishment?
That's like the question about where the US waters end, with the answer being where the sea is at 50 feet deep.
(that's around the minim of what an aircraft carrier needs to safely navigate). The whole thing of what can they do is just a question of how much and how interested are they in doing something, from BTC-E to Megaupload or AlphaBay, there is almost no safe place for anyone as long as they want to shut it down, one exception might be China but probably Binance would be in more trouble there than in the US.
In the end, it's pretty simple, obey the rules and kiss asses the way Coinbase is doing and you're safe, centralized exchanges are and will always be easy prey, if you want to be a rebel switch to true p2p, but you won't make even 1% of the money this way.
Anyhow, I don't think it will be something really serious, yet, this is just an investigation on compliance probably a small fine as a warning for others and Binance will hand over to the IRS their logs to keep relations on good terms.